EP2260969B1 - Coated electrode with steel core and lithium coating - Google Patents

Description

The invention relates to a coated electrode for electric arc welding comprising a central steel core covered with a coating leading to a stable, regular and dynamic arc throughout the welding operation, including during a difficult manual welding operation, such as welding in position, as well as a welding process using such an electrode.

The stability of the electric arc generated during welding with different mixes has been studied, in particular by R. Dedyukh at al, Increasing the stability of the parameters of the process of arc welding with coated electrodes at low current, Welding International vol. 18 n ° 12 pp 963 (2004 ); and G. Datta, Arc-length, arc-voltage and mode of metal transfer in metal-arc welding with coated electrodes, IE (I) Journal-ME vol. 53 pp 181 (1973 ).

EP 1570944 A1 describes a method and an electrode coated with a central metallic core of stainless steel or mild steel. EP 1125673 A1 also describes a method and an electrode coated with a central core of metal or metallic alloy, for example of mild steel.

In these documents, arc stability has been linked to the measurement of the frequency of short circuits observed during welding and their duration.

The short circuits observed during welding are due to the formation of a liquid metallic bridge between the welding bath and the tip of the molten electrode. Their frequency and the evolution of the voltage between each short circuit are linked to the length of the arc: the shorter the arc and the higher this frequency, the less time the arc has to stabilize between two short -circuits. This amounts to continuously observing a voltage slope between two short circuits, indicating that the electric arc does not have time to stabilize and allow time for a drop of molten metal to grow, as illustrated in the Figure 10 .

In the case of a short to very short arc, for example less than 5 mm, each short circuit is synonymous with transfer of metal into the metal bath (f> 6 Hz). For identical or lower frequencies, it is possible to observe short circuits without systematic transfer of metal into the bath.

With regard to the duration of the short circuits, this would depend both on the arc length and on the ease of unhooking the drop of molten metal so that it is transferred into the bath. It is therefore possible to associate this short-circuit phenomenon with the stability of the arc and its dynamism.

However, these documents do not present the means of improving the operating behavior of a coated electrode by playing on its composition.

Indeed, in the field of welding, the energy (E) provided by the consumable during the production of a quality assembly is a very important factor. It is determined by the intensity (I) expressed in amperes (A) of the current used, the voltage (U) expressed in volts (V) between the consumable and the sheet and the welding speed (V) expressed in cm / min according to the following relation: $$\mathrm{E}=\frac{\mathrm{U}\left(\mathrm{V}\right)\times \mathrm{I}\left(\mathrm{AT}\right)}{\mathrm{V}\left(\mathrm{cm}.{\min }^{-1}\right)\times \mathrm{k}}$$

In particular, a problem which arises is to be able to have a coated electrode whose operating behavior has been improved, that is to say allowing to obtain a stable, regular and dynamic arc throughout the welding with said coated electrode, as well as good corrosion resistance of the welded joints thus produced, in particular when they are operated on metal parts used to manufacture structures of the pressurized steam line type for power plants operating fossil or nuclear energy.

In other words, the aim is to provide an improved coated electrode having good arc dynamism allowing the operator easy welding in position with a wide range of current intensity, stability of the welding parameters during all the use of the consumable and sufficient mechanical properties to allow in particular the welding of grade 92 ferritic steels, in particular in compliance with standard ASTM A213 P92, used for their hot mechanical properties in thermal power plants for example, as well as '' good resistance to corrosion.

A solution is then a coated electrode comprising a central steel core at least partially covered with a coating forming a coating around said central core, the central core being in low or non-alloy steel containing iron and less than 5%. by mass of alloying elements, and the coating containing lithium, characterized in that the coating contains from 0.06 to 1% by mass of lithium (% by mass of the coating) and in that the core power plant contains from 0.001 to 4% chromium (% by mass of the core).

Indeed, the combination of a weakly or unalloyed core with a coating which contains lithium, in particular from 0.06 to 1% by mass of lithium, makes it possible to improve the operating behavior of the consumable during its use in welding with l arc, by obtaining a stable, regular and dynamic arc throughout the welding operation, including during a difficult manual welding operation, such as welding in position, it is i.e. on the ceiling, vertically rising, ledge ...

Furthermore, the presence of 0.001 to 4% chromium (% by mass of the core) in the core of the electrode and / or from 5 to 20% chromium in the coating (% by mass of the coating) provides resistance to corrosion of the weld joint obtained.

• l'âme centrale comprend de 0.001 à 3% de chrome (% en masse de l'âme), de préférence moins de 2,5% de chrome.
• l'enrobage contient de 5 à 20% de chrome (% en masse de l'enrobage).
• les éléments d'alliage de l'acier allié sont typiquement C, Mn, Si, Cr, Ni, Co, Mo, Nb, V, B, W et/ou N.
• l'électrode a une longueur comprise entre 30 et 45 cm, et un diamètre d'âme compris entre 1.6 et 6.3 mm, généralement entre 2.5 et 5.0mm.
• l'électrode contient de 0.02 à 0.30 % en masse de lithium par rapport à la masse totale de l'électrode, c'est-à-dire âme et enrobage.
• l'électrode contient du cobalt en une proportion allant jusqu'à 2 % par rapport au poids de l'électrode enrobée (i.e. enrobage et âme), de préférence 0.1 à 1.5% de cobalt. Avantageusement, l'âme contient jusqu'à 1,5% (% en masse de l'âme). En effet, l'addition de colbalt (Co) dans le métal déposé est bénéfique car elle permet d'augmenter la valeur de la température de transformation AC1 dudit métal déposé, ce qui offre une marge de sécurité accrue pour l'application d'un traitement thermique post-soudage.
• l'électrode contient de 0.9 à 2% de tungstène, de 0.001 à 0.1% de bore et/ou de 0.01 à 0.5% de niobium. Les éléments W, B et Nb sont incorporés à l'électrode, en particulier à l'enrobage, afin de conférer des propriétés de résistance au fluage au métal déposé. Si la balance de ces éléments n'est pas bien respectée ou si un de ces éléments est manquant, les propriétés de résistance au fluage s'en trouvent fortement dégradées.
• le lithium (Li) compris dans l'enrobage est sous la forme de silicate de Li, de carbonate de Li, de feldspath de Li, de fluorure de Li, de chlorure de Li, de titanate de Li ou d'oxyde de Li
• l'âme centrale comprend (% en masse de l'âme) de 0.01 à 0.15% de carbone, au plus 0.5% de silicium, au plus 1.5% de manganèse, au plus 0.02% de phosphore, au plus 0.01% de soufre, de 0.001 à 4% de chrome, au plus 1% de molybdène, au plus 1% de nickel, de 0.001 à 0.04% d'aluminium, au plus 1.5% de cobalt, au plus 0.1% de niobium, au plus 0.5% de vanadium, au plus 2.5% de tungstène, au plus 0.1% de bore, au plus 0.1% d'azote et/ou de 90 à 99.9% de fer.
• l'âme centrale comprend (% en masse de l'âme) de 0.01 à 0.10% de carbone, au plus 0.2% de silicium, au plus 1 % de manganèse, au plus 0.01% de phosphore, de 0.001 à 0. 1% de chrome, au plus 0.1% de molybdène, au plus 0.1% de nickel, au plus 0.01% d'aluminium, au plus 0.01% de cobalt, au plus 0.01% de niobium, au plus 0.05% de vanadium, au plus 0.1% de tungstène, au plus 0.01% de bore et/ou au plus 0.01% d'azote.
• l'électrode contient (% en masse de l'électrode) de 0.01 à 2% de carbone, de 0.5 à 4% de silicium, de 1 à 2.5% de manganèse, de 5 à 11% de chrome, de 0.1 à 1% de molybdène, de 0.1 à 1% de nickel, de 0.1 à 2% de cobalt, de 0.01 à 0.5% de niobium, de 0.1 à 0.5% de vanadium, de 0.9 à 2% de tungstène, de 0.001 à 0.1% de bore, de 0.01 à 0.1% d'azote, de 0.02 à 0.3% de lithium, de 0.01 à 0.5% de sodium, de 0.01 à 0.7% de potassium, de 5 à 15% de calcium, de 1 à 10% de fluor et/ou de 40 à 80 % de fer.
• l'enrobage peut contenir, en outre, un ou des carbonates de calcium, un ou des éléments d'alliages, un ou des éléments métalliques, du spath fluor, etc...

Depending on the case, the electrode of the invention may include one or more of the following characteristics:

• the central core comprises from 0.001 to 3% chromium (% by mass of the core), preferably less than 2.5% chromium.
• the coating contains 5 to 20% chromium (% by mass of the coating).
• the alloying elements of the alloyed steel are typically C, Mn, Si, Cr, Ni, Co, Mo, Nb, V, B, W and / or N.
• the electrode has a length between 30 and 45 cm, and a core diameter between 1.6 and 6.3 mm, generally between 2.5 and 5.0mm.
• the electrode contains from 0.02 to 0.30% by mass of lithium relative to the total mass of the electrode, that is to say core and coating.
• the electrode contains cobalt in a proportion of up to 2% relative to the weight of the coated electrode (ie coating and core), preferably 0.1 to 1.5% of cobalt. Advantageously, the core contains up to 1.5% (% by mass of the core). Indeed, the addition of colbalt (Co) in the deposited metal is beneficial because it makes it possible to increase the value of the transformation temperature AC1 of said deposited metal, which offers an increased safety margin for the application of a post-weld heat treatment.
• the electrode contains 0.9 to 2% tungsten, 0.001 to 0.1% boron and / or 0.01 to 0.5% niobium. The elements W, B and Nb are incorporated in the electrode, in particular in the coating, in order to impart creep resistance properties to the deposited metal. If the balance of these elements is not respected or if one of these elements is missing, the creep resistance properties are greatly degraded.
• the lithium (Li) included in the coating is in the form of Li silicate, Li carbonate, Li feldspar, Li fluoride, Li chloride, Li titanate or Li oxide
• the central core comprises (% by mass of the core) of 0.01 to 0.15% of carbon, at most 0.5% of silicon, at most 1.5% of manganese, at most 0.02% of phosphorus, at most 0.01% of sulfur, from 0.001 to 4% chromium, at most 1% molybdenum, at most 1% nickel, from 0.001 to 0.04% aluminum, at most 1.5% cobalt, at most 0.1% niobium, at most 0.5% vanadium, at most 2.5% tungsten, at most 0.1% boron, at most 0.1% nitrogen and / or 90 to 99.9% iron.
• the central core comprises (% by mass of the core) from 0.01 to 0.10% of carbon, at most 0.2% of silicon, at most 1% of manganese, at most 0.01% of phosphorus, from 0.001 to 0. 1% chromium, at most 0.1% molybdenum, at most 0.1% nickel, at most 0.01% aluminum, at most 0.01% cobalt, at most 0.01% niobium, at most 0.05% vanadium, at most 0.1% tungsten, at most 0.01% boron and / or at most 0.01% nitrogen.
• the electrode contains (% by mass of the electrode) of 0.01 to 2% of carbon, of 0.5 to 4% of silicon, of 1 to 2.5% of manganese, of 5 to 11% of chromium, of 0.1 to 1% molybdenum, 0.1 to 1% nickel, 0.1 to 2% cobalt, 0.01 to 0.5% niobium, 0.1 to 0.5% vanadium, 0.9 to 2% tungsten, 0.001 to 0.1% boron , 0.01 to 0.1% nitrogen, 0.02 to 0.3% lithium, 0.01 to 0.5% sodium, 0.01 to 0.7% potassium, 5 to 15% calcium, 1 to 10% fluorine and / or from 40 to 80% iron.
• the coating may also contain one or more calcium carbonates, one or more alloying elements, one or more metallic elements, fluorspar, etc.

The invention also relates to a method of electric arc welding of one or more metal parts using a coated electrode according to the invention.

• la ou les pièces à souder sont en un acier faiblement et/ou fortement allié contenant de 3 à 20 % en masse d'éléments d'alliage, y compris des aciers dissimilaires, par exemple un acier grade 23 ou 24 avec un acier grade 92 dans le cadre d'un assemblage dissimilaire.
• le soudage est opéré à plat, au plafond, en verticale montant ou en corniche.

Depending on the case, the method of the invention may include one or more of the following characteristics:

• the part or parts to be welded are made of low and / or high alloy steel containing from 3 to 20% by mass of alloying elements, including dissimilar steels, for example a grade 23 or 24 steel with a grade 92 steel as part of a dissimilar assembly.
• welding is carried out flat, on the ceiling, vertically rising or on a ledge.

In fact, in the context of the present invention, the composition of the coated electrode was chosen so as to obtain an improved operating behavior compared to a standard electrode, as well as a solder joint having good corrosion resistance. . In doing so, it has been noticed, as detailed below, that by adding lithium in the coating of the electrode and chromium in the core, the above problem was solved.

One of the means of incorporating lithium in the coating is based on the use of a suitable silicate, namely a lithium silicate. Thus, aqueous lithium silicates can serve as a binder for the manufacture of coating pastes for electrodes. If sodium and potassium silicates have been widely used to make welding consumables, lithium silicate has rarely been. In addition, the use of lithium has never been recommended to improve the operating behavior of a coated electrode.

More specifically, in the context of the present invention, an electrode has been developed which makes it possible to weld in a greater current intensity range than a standard electrode and which makes it possible to keep the welding parameters stable during the operation. .

The arc stability and the dynamism of the consumable of the invention have been improved by judiciously choosing the composition of the metallic core, namely a low or unalloyed steel, that is to say comprising less than 5% d alloying elements, for example steel type NFA 35/055 (AFNOR standard).

Thus, one can use a coated electrode having a chromium content of 0.001 to 4% in its core and a lithium content between 0.06 and 1% in the coating. This also makes it possible to increase the operating range of the coated electrode obtained.

In fact, as already said, the introduction of a minimum quantity of the lithium element (Li) in the form of Li silicate, Li carbonate, Li fluoride, Li chloride, Li feldspar, ..., in the coating provides a greater arc dynamism, which results in a clearer bath, better clearance of the slag, better wetting of the cord.

Overall, this makes it possible to obtain a coated electrode giving a better quality result than the known coated electrodes.

In addition, the combination of this element Li with the fact of using a so-called “synthetic” electrode since it comprises a central core of low or non-alloy steel (ie with less than 5% by weight of alloy elements) but intended to be used for the welding of highly alloyed steels, that is to say comprising more than 5% by weight of alloying elements, such as C, Mn, Si, Cr, Ni, Co, Mo, Nb , V, B, W, N, makes it possible to obtain a coated electrode which can weld over a wider range of intensity while maintaining very good behavior and stability of the welding parameters.

• La Figure 1 est un enregistrement des paramètres de tension et d'intensité lors du soudage avec une électrode homogène,
• La Figure 2 est similaire à la Figure 1 mais obtenue avec une électrode synthétique contenant moins de 0.06% de Li dans l'enrobage,
• La Figure 3 est similaire aux Figures 1 et 2 mais obtenue avec une électrode synthétique contenant 0.28% de Li dans l'enrobage,
• Les Figures 4 à 9 sont des photographies montrant les profils de cordons de soudage obtenus,
• La Figure 10 est un enregistrement de la tension d'une électrode lors de son soudage,
• La Figure 11 schématise la répartition de la durée des courts-circuits lors du soudage avec une électrode entière,
• La Figure 12 montre les répartitions des durées de court-circuit d'une électrode homogène contenant 0.15% de Li dans l'enrobage en fonction du temps de soudage,
• Les Figures 13 et 14 représentent les répartitions des durées de court-circuit d'une électrode synthétique contenant moins de 0.06% de Li dans l'enrobage (Fig. 13) ou contenant 0.28% de Li dans l'enrobage (Fig. 14) en fonction du temps de soudage, et
• La Figure 15 schématise une structure d'électrode enrobée.

The invention will now be illustrated by the following comparative examples given and the appended figures among which:

• The Figure 1 is a record of the voltage and current parameters when welding with a homogeneous electrode,
• The Figure 2 is similar to the Figure 1 but obtained with a synthetic electrode containing less than 0.06% of Li in the coating,
• The Figure 3 is similar to Figures 1 and 2 but obtained with a synthetic electrode containing 0.28% Li in the coating,
• The Figures 4 to 9 are photographs showing the profiles of weld beads obtained,
• The Figure 10 is a record of the voltage of an electrode during its welding,
• The Figure 11 diagrams the distribution of the duration of short-circuits during welding with an entire electrode,
• The Figure 12 shows the distribution of the short-circuit durations of a homogeneous electrode containing 0.15% Li in the coating as a function of the welding time,
• The Figures 13 and 14 represent the distributions of the short-circuit durations of a synthetic electrode containing less than 0.06% of Li in the coating ( Fig. 13 ) or containing 0.28% Li in the coating ( Fig. 14 ) as a function of the welding time, and
• The Figure 15 schematically shows a coated electrode structure.

Examples

As part of the tests carried out and recorded below, a Stubby type welding clamp of the Weldline ™ brand was used, provided with a coated electrode, supplied with electric current by a DC current generator (Mega Arc type). 5 sold by the company Oerlikon ™ or of type Digiwave 400A of the brand SAF-FRO ™.

Typically, a coated electrode 1 is composed, as illustrated in Figure 15 a central core 2 of steel covered with a solid coating 3, forming a coating around said central core 2. Such an electrode generally has a length of between 30 and 45 cm, and a core diameter of between 1.6 and 6.3 mm, generally between 2.5 and 5.0mm.

• des électrodes dites « homogènes » dont l'enrobage contient plus de 0.06% en masse de Li par rapport à l'enrobage,
• des électrodes dites « synthétiques » dont l'enrobage contient moins de 0.06% en masse de Li par rapport à l'enrobage,
• des électrodes dites « synthétiques » dont l'enrobage contient plus de 0.06% en masse de Li par rapport à l'enrobage.

During the tests, three types of electrodes were tested in grade 92 arc welding, namely:

• so-called “homogeneous” electrodes, the coating of which contains more than 0.06% by mass of Li relative to the coating,
• so-called “synthetic” electrodes, the coating of which contains less than 0.06% by mass of Li relative to the coating,
• so-called “synthetic” electrodes, the coating of which contains more than 0.06% by mass of Li relative to the coating.

A “homogeneous electrode” is a coated electrode whose chemical composition of the metallic core is very close to that of the deposited metal (which can be close to that of the base metal). In this case, the coating is composed of mineral elements whose decomposition in the electric arc makes it possible to release ionizing elements from the arc, to form the protective gas of the bath, such as CO and CO ₂ , and the slag covering the cord. The latter also contains some alloying elements allowing the chemical composition of the deposited metal to be adjusted more finely.

In the case of welding of grade 92 steels, the core has for example the chemical composition given in Table 1.

Furthermore, a “synthetic electrode” is an electrode whose chemical composition of the core differs significantly, on one or more elements, from the chemical composition of the deposited metal. By way of example, this amounts to using a core considered to be weakly or not alloyed for welding a steel considered to be highly alloyed.

In the case of “synthetic” electrodes, the alloying elements not being present in the core and which are found in the deposited metal come from the coating. The latter is then composed of mineral elements, the decomposition of which in the electric arc makes it possible to release ionizing elements from the arc, to form the protective gas of the bath and the slag covering the cord, and of mono-elementary metallic powders ( Cr metal, Ni metal, Mn metal, ...) or poly-elementals (ferro-alloys, metal oxides, ...).

In the case of welding of grade 92 steels, the core has for example the chemical composition presented in Table 2.

Welding tests were therefore carried out using these 3 types of consumables whose chemical compositions of the cores corresponded to those called “% by typical mass” as given in Tables 1 and 2.

The coatings were as defined above. In other words, the so-called "synthetic" electrodes differ from each other only by the lithium content they contain.

It was first noted that the use of a homogeneous consumable had a major drawback. Indeed, manual welding with this electrode shows an evolution of the voltage during welding.

So the figure 1 represents a recording of the voltage and current parameters obtained during welding with a homogeneous electrode containing 0.15% by mass of Li in the coating (i.e. 0.06% by mass of Li in the entire electrode), with a diameter of 2.5 mm in flat position, at an intensity of 87 A, or approximately 90% of the maximum recommended intensity.

It appears that after 30 seconds of welding, the electrode begins to blush. The behavior of the consumable changes and the drop transfer becomes coarser. This is characterized, on recording, by a drop in voltage and an increase in intensity. This then generates variations in the welding parameters, causing disturbances to the arc, which can result in a modification of the welding energy, as well as a variation in the transfer of the alloying elements in the molten bath, which generates vagaries in terms of homogeneity of the welding results obtained with such a coated electrode.

From there, such an electrode can only be used in a necessarily limited operating range, that is to say ranging from 70 to 85 A for example for a coated electrode called with a diameter of 2.5 mm, when it is desired to be certain to obtain a bead without any variation in the welding parameters.

Conversely, as shown by Figures 2 and 3 , the use of a synthetic coated electrode makes it possible to obtain stable voltage and intensity parameters throughout the welding, whether or not it contains more than 0.06% by mass of lithium in the coating.

The use of a weakly or unalloyed core (i.e. less than 5% by mass of alloying elements) therefore prevents the consumable from heating up during welding and guarantees the stability of the voltage and intensity parameters.

This difference in behavior is explained by the difference in conductivity that exists between an alloyed steel (more than 5% of alloying elements) and a low or non-alloyed steel thus leading to very good stability of the welding parameters.

In addition, the addition of lithium in the coating of a synthetic coated electrode with a diameter of 2.5 mm for example, at a height of at least 0.06% by mass of the coating of the electrode, makes it possible to increase the range d operating intensity of the consumable from the range 80-100 A to the range 70-100 A.

Indeed, lithium allows better operational performance and the welding of a bead with a lower intensity and therefore a lower welding energy.

It should be noted that this difference of approximately 10 A also makes it possible to have an operating behavior of the coated electrode greater than the same operating intensity as a synthetic electrode containing an amount of lithium of less than 0.06% by mass in l 'coating.

The figures 4 to 9 are photographs showing the profiles (sections) of welding seams obtained by welding with the coated electrodes in a vertical rising position with parameters of 82 A and 21 V. These sections of seam are presented two by two to show the evolution of wetting depending on the consumption of the electrode.

As seen on the Figure 4 , with the “homogeneous” electrode used, which contains 0.15% by mass of Li in the coating, a flat bead is obtained, therefore good wetting, at the start of welding.

However, we have observed a change in behavior of the consumable during welding which is associated with heating of the highly alloyed core of the electrode and which has the effect of modifying the drop transfer during welding, thus giving a rather rounded bead at the end of the welding, as shown in Figure 5 .

The Figures 6 and 7 show the forms of beads obtained under the same operating conditions, at the start and end of welding respectively, with a “synthetic” electrode containing less than 0.06% by mass of Li in the coating.

We can see on the Figure 6 that the cord is slightly curved from the start and that the behavior of the electrode does not vary during the operation since no heating of the unalloyed core is visible on the Figure 7 .

The Figures 8 and 9 relate to the use of a “synthetic” electrode containing 0.28% by mass of Li in the coating instead of only 0.06% for the electrode used previously. We can see on the Figures 8 and 9 that this only difference with the previous electrode ( Fig. 6 and 7 ) provides better wetting thanks to the more dynamic behavior of the arc. This wetting remains identical during the entire welding operation (start of welding: Fig. 8 ; end of welding: Fig. 9 ) showing that the absence of heating of the non-alloyed core makes it possible to keep constant welding parameters.

These tests demonstrate the advantage of a “synthetic” electrode, the central core of which is made of low-alloy or non-alloy steel and the coating of which contains lithium, preferably at least 0.06% by mass of lithium in the coating, advantageously from 0.06 to 0.5% by mass of lithium.

Additional tests have highlighted the influence of lithium on the dynamism and arc stability of the electrode. The “dynamism of a consumable” is characterized by its ease in detaching the drops of metal when it melts in the arc.

It is therefore advisable to be interested in the duration of the short-circuits and also in their frequency during welding. So the Figure 10 illustrates a recording of the voltage parameter of an electrode during its welding. In fact, the shorter the short circuit, the easier it is to detach the drop and the more dynamic the electrode is considered. This type of electrode very often exhibits better behavior in position, dynamism being synonymous with good clearance of slag and flat cord.

The voltage parameter was therefore recorded for the three types of electrode mentioned above at a frequency of 30 kHz in order to be able to measure the short-circuit times and see their distribution according to the consumable.

The Figure 11 presents the distribution of the short-circuit durations obtained from these records. The distribution is carried out as a cumulative percentage in order to be able to easily determine the proportion of short circuits present in a given time range. It is noted that the electrodes having a Li concentration> 0.06% by mass in their coating have a higher proportion of short-circuits of duration less than 4.2 ms. Values of 30% and 43% are found respectively.

This difference clearly indicates that the detachment of the drops of molten metal is easier when the concentration of Li is greater than 0.06% by mass in the coating of the consumable whatever the core used to manufacture the coated electrode.

In addition, Figures 12 to 14 represent the evolution of the distribution of short-circuit durations as a function of the part of the welded consumable, that is to say melted. The two parts called "half 1 ^st welding" and "2 ^nd half of welding" are determined by dividing the welding time in half.

The Figure 12 shows that the distribution of the short-circuit durations in the case of a homogeneous electrode varies by 33% between the first and the second part. This phenomenon can be linked to the heating of the core inducing an easier melting of the consumable and consequently, a formation of large drops more difficult to detach. It is said that the electrode runs out of steam. This is confirmed by the short-circuit frequency values which are 11 Hz in the first half of the welding, therefore with a very short and very stable arc, and with good transfer of molten metal, and only 5 Hz in the second half of the welding, therefore leading to a short and less stable arc, each short circuit does not necessarily correspond to a transfer of molten metal.

The Figures 13 and 14 clearly show that, in the case of a synthetic consumable, the distribution of the short-circuit durations changes very little between the start and the end of the welding.
There are then differences in distribution of 6.9% and 6.5%, respectively, for a synthetic electrode containing less than 0.06% by mass of Li and for a synthetic electrode containing 0.28% by mass of Li in the coating. This stability during welding is explained by the use of a weakly or unalloyed core.

The stability of the short-circuit frequency between the two parts of each electrode should also be noted. Values of 9 Hz and 7 Hz, respectively, were determined for 0.28% and less than 0.06% by mass of Li in the coating.

The presence of Li also has the consequence of shortening the electric arc and thus making it more rigid, therefore more directive, and more stable with a more regular transfer of molten metal into the bath throughout the welding operation.

A Cr content greater than 3% is necessary to give the deposited metal sufficient corrosion resistance, in particular for grade 92 applications where the deposited metal must contain between 8 and 10% (% by mass) of Cr for suitably resist pressure corrosion induced by water vapor. Controlling this content is important because Cr and other ferrite delta phase forming elements such as the elements Nb, W, V, Si, Mo can induce a high susceptibility to hot cracking. In order to avoid this phenomenon, and as is the case for this coated electrode, the balance of these elements is controlled.

The insertion of Co into the deposited metal makes it possible to increase the value of the transformation temperature AC1 of said deposited metal and thus offers an increased safety margin for the application of a post-welding heat treatment.

The elements W, B, Nb and N are inserted into the consumable in order to confer creep resistance properties on the deposited metal. If the balance of these elements is not respected or if one of these elements is missing, the creep resistance properties are greatly degraded.

The electrodes coated according to the present invention are particularly well suited to the welding of highly alloyed steel, that is to say containing more than 5% by mass of alloying elements, for example steels of the grade 92 type.

Claims (10)

1. Coated electrode (1) comprising a central steel core (2) at least partly covered with a coating (3) forming a coating around said central core (2), the central core being low-alloy or non-alloy steel containing iron and less than 5% by weight of alloying elements, and the coating containing lithium, characterized in that the coating contains from 0.06 to 1% by weight of lithium and in that the central core comprises 0.001 to 4% chromium (% by weight of the core).
2. Coated electrode according to the preceding claim, characterized in that the central core comprises 0.001 to 3% chromium (% by weight of the core), preferably less than 2.5% chromium.
3. Coated electrode according to one of the preceding claims, characterized in that the lithium (Li) included in the coating is in the form of Li silicate, Li carbonate, Li feldspar, Li fluoride, Li chloride or Li oxide
4. Coated electrode according to one of the preceding claims, characterized in that the central core comprises (% by weight of the core) of 0.01 to 0.15% carbon, at most 0.5% silicon, at most 1.5% of manganese, not more than 0.02% phosphorus, not more than 0.01% sulfur, 0.001 to 4% chromium, not more than 1% molybdenum, not more than 1% nickel, 0.001 to 0.04% aluminum, not more than 1.5 % of cobalt, not more than 0.1% of niobium, not more than 0.5% of vanadium, not more than 2.5% of tungsten, not more than 0.1% of boron, not more than 0.1% of nitrogen and / or of 90 to 99.9% of iron.
5. Coated electrode according to one of the preceding claims, characterized in that the central core comprises (% by weight of the core) of 0.01 to 0.10% carbon, at most 0.2% silicon, at most 1% of manganese, not more than 0.01% phosphorus, 0.001 to 0.1% chromium, not more than 0.1% molybdenum, not more than 0.1% nickel, not more than 0.01% aluminum, not more than 0.01% cobalt, not more than 0.01% of niobium, not more than 0.05% of vanadium, not more than 0.1% of tungsten, not more than 0.01% of boron and / or not more than 0.01 % of nitrogen.
6. Coated electrode according to one of the preceding claims, characterized in that it contains (% by weight of the electrode) of 0.01 to 2% carbon, 0.5 to 4% silicon, 1 to 2.5% of manganese, 5 to 11% chromium, 0.1 to 1% molybdenum, 0.1 to 1% nickel, 0.1 to 1% cobalt, 0.1 to 0.5% niobium, 0.1 to 0.5% vanadium, 0.9 to 2% tungsten, 0.01 to 0.1% boron, 0.01 to 0.1% nitrogen, 0.01 to 0.3% lithium, 0.01 to 0.5% sodium, 0.01 to 0.7% potassium, 5 to 15% calcium, 1 to 10% fluorine and / or 40 to 80% iron.
7. Coated electrode according to one of the preceding claims, characterized in that the alloying elements of the alloy steel are selected from the group consisting of C, Mn, Si, Cr, Ni, Co, Mo, Nb, V , B, W and N.
8. A method of electric arc welding of one or more metal parts using a coated electrode according to one of the preceding claims.
9. A welding method according to claim 8, characterized in that the part or parts to be welded are a low and / or high alloy steel containing from 3 to 20% by weight of alloying elements.
10. A welding method according to claim 8, characterized in that the welding is executed on the floor, on the ceiling, on a vertical rising or on a cornice.

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